| OCR Text |
Show reduce the minimum boiler load down to 15 % without oil fuel consumption. As illustrated in Fig.1, the normal range of the load is from 43 % to 100 %. Our objective is to widen the operating range of the burner load preferably less than 15 % by mounting a condensing module inside a burner. Under 43 % burner load, the coal concentration in a fuel pipe is lean because the carrying primary air flow rate cannot be reduced due to coal sedimentation in the fuel pipe. For example, at 15 % burner load, CIA [coal feed rate/carrying air flow rate] is about O. 08, as shown in Fig .1. So the technology is required to establish the stable ignition in such a lean coal concentration condition. If the minimum burner load is reduced to 15 % , the minimum boiler load of 15 % will be established without burner cut off nor oil fuel back up. Such a burner system is excellent on DSS operation in coal firing boilers, because the burner-cut-off accompanied by the mill-cut-off is not desirable even in a low load operation. We have encouraged the new combustion procedure of wide load ral1ge running burners since 1987 to establish the flame stability technology (boiler running load range is from 15% to 100% without backup fuel). Consequently, a new wide load range burner has been designed in which a condensing module called a concentrator is mounted. This concentrator is designed to be fitted in the conventional type coal firing burner as well. Fig.2 shows the cross section of the wide load range burner with an inner coal concentrator constructed with three segments, plug cylinder and corn. Features of the new burners are as follows. l)Stable ignition at extremely low load running. 2)Low pressure drop at the concentrator when the burner is on high load range running conditions. The present paper reveals the fundamental and practical examinations we did for designing a new burner sustaining stable ignition in a wide range of load. FUNDAMENTAL STUDIES 1. Ignition Model of Coal Particles 1.1 Theoretical The model burner is a cylinder in which the flow of coal and gas mixture is one-dimensional along the axis as shown in Fig.3. The coal particles inside the burner are warmed by convection and radiation from the flame. The particles start to burn at the burner exit at which their temperature reaches to the ignition point. At steady state, the flame front is apparently stationary at the burner exit as the axial velocity of gas is just equal to the flame propagation velocity. Take a heat balance for the unburnt particles. That is, M C (dT /dt) = Z - Q o ps s ( 1 ) 2 |
